1. Field of the Invention
This invention relates to photoactive coatings and, in one aspect, to methods of changing or obtaining the phase of a material, e.g., an anatase crystalline phase of an oxide of titanium, such as titanium dioxide, from an amorphous phase of titanium oxide or from titanium metal. In another aspect, the invention relates to methods of obtaining a photoactive, e.g., photoactively hydrophilic and/or photocatalytic coating, and/or to articles made thereby.
2. Technical Considerations
For many substrates, e.g., glass substrates such as architectural windows, automotive transparencies, and aircraft windows, it is desirable that the surface of the substrate is substantially free of surface contaminants, such as common organic and inorganic surface contaminants, for as long a duration as possible. Traditionally, this has meant that these surfaces are cleaned frequently. This cleaning operation is typically performed by manually wiping the surface with or without the aid of chemical cleaning solutions. This approach can be labor, time, and/or cost intensive. Therefore, a need exists for methods to clean glass substrates that reduce the frequency and/or need for such manual cleaning operations.
It is known that certain semiconductor metal oxides provide a photoactive (hereinafter “PA”) coating. The terms “photoactive” or “photoactively” refer to the photogeneration of a hole-electron pair when illuminated by electromagnetic radiation of a particular frequency, typically ultraviolet (“UV”) light. Above a certain minimum thickness, these PA coatings are typically photocatalytic (hereinafter “PC”). By “photocatalytic” is meant a coating which upon exposure to certain electromagnetic radiation, such as UV, interacts with organic contaminants on the coating surface to degrade or decompose the organic contaminants. With sufficient PC activity, these PC coatings are also self-cleaning. By “self-cleaning” is meant having sufficient PC activity to decompose organic contaminants fast enough that manual wiping to remove organic contaminants is not required. In addition, PC coatings are also typically hydrophilic. By “hydrophilic” is meant water wetting with a contact angle with water of generally less than 20 degrees. The hydrophilicity of the PC coatings helps reduce fogging, i.e., the accumulation of water droplets on the coating, which may decrease visible light transmission and visibility through the coated substrate.
Titanium dioxide (TiO2) coatings are known to have hydrophilic and/or self-cleaning properties. However, not all phases of titanium dioxide are acceptable for providing self-cleaning and/or hydrophilic coatings. It is currently preferred to use the anatase crystalline phase rather than the amorphous phase or rutile crystalline phase of titanium dioxide to form PC coatings.
Sputter coating titanium dioxide, e.g., as a protective overcoat, has been used and is disclosed in U.S. Pat. No. 4,716,086. A limitation of conventionally sputter depositing titanium dioxide is that the anatase crystalline phase is not obtained. Another limitation is that sputter depositing a metal film is more efficient than depositing a metal oxide film. In the instance where a metal oxide film is desired, an efficient method is to sputter deposit a metal film on a substrate, and thereafter heat the deposited metal film in air. In the case of sputter deposited titanium metal film, the oxide film formed after heating is usually not the anatase phase but rather the rutile phase of titanium dioxide. Publications directed to the formation of titanium dioxide coatings on a glass substrate include U.S. Pat. Nos. 5,595,813 and 6,027,766, and “Photooxidative Self-cleaning Transparent Titanium Dioxide Films on Glass”, Paz et al., J. Mater. Res., Vol. 10, No. 11, pp. 2842-48 (November 1995).
Another known method of depositing metal oxide films is by pyrolytic deposition, e.g., spray pyrolysis. Pyrolytic deposition has some advantages over MSVD deposition. For example, spray pyrolysis typically is less costly and poses fewer environmental concerns than MSVD processes. However, materials such as titania typically show little if any photocatalytic activity when pyrolytically deposited directly onto a glass substrate.
As can be appreciated, it would be advantageous to provide a method of making a pyrolytically deposited hydrophilic and/or photocatalytic coating, e.g., a titanium dioxide-containing coating.